As is known to persons skilled in the art, a ceramic object is normally formed by a body consisting of clay material and by a vitreous surface coating specially formulated to provide the object itself, after suitable firing at a high temperature, with improved physical and aesthetic characteristics.
It is also known that the vitreous coating is obtained by applying ceramic glazes consisting of finely ground mixtures of the following components: (a) frit consisting of borosilicate glass of varying composition which, during firing of the ceramic object, fuses and forms a compact layer; (b) pigments, consisting of inorganic substances with coloring power, which are stable at the temperature at which the ceramic object is fired and are enclosed by the vitreous layer to which they impart the desired chromatic characteristics; (c) suspending agents, that are clays, kaolins and other substances necessary for obtaining, in wet glazes, rheological characteristics suitable for the application; finally, it may be necessary to use other specific inorganic substances in order to obtain special effects in the glaze, such as opacification, special surface features, etc.
In the case of wet glazing, the ceramic glazes are normally ground in HALSING mills, to which water is added, thereby producing a liquid suspension or glaze slurry which can be applied to the ceramic object using various methods, among which the following are mentioned: (1) spraying: with this method the glaze is atomized by means of nozzles and compressed air or a pressure pump, the jet being directed onto the surface to be coated; (2) dipping: with this method the object to be coated is dipped into the glaze slurry, thereby ensuring that its entire surface is glazed; (3) bell-chamber method: in this case, the glaze slurry is made to flow continuously over a bell-formed body, in the form of a vertical film, while the object to be coated moves horizontally below it. This system is suitable only for coating flat surfaces; (4) orifice method: this is a method similar to the preceding one, but the film of glaze is obtained by means of a hopper with an adjustable horizontal slit; (5) disk method: according to this method, a system of rotating disks is used, which produces a "shower" of glaze. This method is suitable for the formation of non-uniform layers and is used, in particular, for glazes with a low water content.
Industrially, for mass production, use is made of continuous-application machines which recover and recycle the excess glaze not retained by the object to be glazed.
As mentioned above, wet glazing has various drawbacks which, according to their nature, can be divided into three categories: (a) qualitative, (b) energy-related and (c) ecological. As regard the first type of drawback, it should be noted that, during application, the rheological characteristics of the circulating ceramic slurry tend to vary, especially owing to water evaporation, thereby requiring constant monitoring and restoration of the optimum conditions.
With respect to the second type of drawback, before firing in the kiln, the object glazed with wet glaze must be dried, thus resulting in thermal energy being applied (i.e. the consumption of energy). In addition, the glaze application and/or purification of the waste and sludge, which will be described below, require further consumption of energy.
As regards, finally, the third type of drawback, it should be noted that, when changing the product, it is often necessary to water-wash the mill, the conveying and storage equipment, as well as the glazing machines. Quantities of glaze ranging from 1% to 10% are thus lost, depending on the duration of the production compaign. Furthermore, these washing operations give rise to liquid effluents which must be purified before being discharged. Typical pollutants are heavy metals and boron, the removal of which, especially in the case of the latter, is particularly costly.
Purification of water and waste gives rise to the problem of sludge which, due to the presence of heavy metals, is normally classified as harmful toxic waste. Where recycling to production is not possible, which is in any case difficult to implement the said sludge must be disposed of a required by law.
In order to overcome the drawbacks of wet glazes, attempts have been made to apply dry glazes in powder form, using various methods, but these have also produced unsatisfactory results, in particular owing to the lack of uniformity of the layer applied, inadequate adhesion to the ceramic object and environmental problems due to the volatility of the powder.
For some years, however, there has been an increasing trend towards products in the form of granules, pellets and flakes, suitable for continuous glazing processes on flat and horizontal surfaces. Application is effected by means of feeder machines. These machines normally consist of: a hopper for charging the material, the bottom of which is provided with an adjustable slit; (2) a feeding system consisting of a vibrating sieve with a wide-mesh screen or of two or more rotating rollers. Both the sieve and the rollers have the function of distributing uniformly the glaze on the (flat) objects to be coated, which pass directly underneath; (3) a system for recovering and recycling the glaze not used: this system consists of a collection hopper and a bucket elevator, which brings the recovered glaze back to the charging hopper; (4) if necessary, a dust extractor with a dust-removal filter, for preventing the dust from escaping.
As is known, pelletized glazes are obtained from conventional ground glazes. Through the addition of binders, granulation in a special mixer and drying, spheroidal pellets of a certain consistency are obtained. Usually screening is required in order to ensure a constant granulometric range.
Again using conventional glazes, it is possible to prepare sintered coarse agglomerates in which cohesion between the particles is developed by keeping the product for a certain period at temperatures close to the softening range. These sintered product must then be reduced into granules of the required dimensions by means of grinding and granulometric grading.
Frit granules, however, are obtained by means of roller milling and grading into the required granulometric ranges. It is also possible for them to be colored using inorganic pigments.
Finally, there are laminated frits which are made from frit flakes by milling and screening.
While not possessing the negative features of wet glazes and dry glazes in powder form, granulated enamels have other disadvantages peculiar to them.
All the granulated products are supplied ready for use and applied directly. Any contamination and lack of homogeneity, resulting from production plants and cycles which are sometimes complex, are not subject to the dispersive action of the mill, as in the case of wet glazes; there is consequently more reject of the ceramic product.
As already mentioned above existing granulated products require screening or grading during production. This result in the formation of rejects or various granulometric fractions, thereby increasing the cost of recycling during production, if they are not sold in proportionate quantities.
Furthermore, the preparation of pelletised glazes involves various operations which are certainly more complex than those required by wet glaze. The final cost of the product is bound to be higher. Finally, it must be remembered that, depending on the type of granules and the production process, the shape of the granules is always the same and that only the average dimensions may vary according to the various granulometric fraction.
Therefore, the object of the present invention is to produce a new type of ceramic glaze which does not have the drawbacks either of wet ceramic glazes or of other types of dry glazes known hitherto.